In recent years, the drive for energy efficiency and low cost motors has led to development of many types of electric motors and generators for various applications. Among the electric motors, permanent magnet synchronous motors (PMSM) are known to have high power density and efficiency. An interior permanent magnet (IPM) motor, which is a specific type of PMSM and also called permanent magnet reluctance (PMR) motor because of its hybrid ability to produce reluctance torque as well as permanent magnet torque, is one of the most prevalent types. A PMR motor includes a rotor having one or more permanent magnets embedded therein and generates a higher torque than a motor with a surface mounted magnet rotor. It is because the permanent magnets are positioned in the rotor core in such a way as to provide saliency in the magnetic circuit in the rotor core, which produces an additional reluctance torque.
During operation, each magnet embedded in the rotor core is subject to centrifugal force. In order to retain the magnets within the rotor core under the centrifugal force and also minimize flux leakage to other poles within the rotor core, narrow sections of rotor core material are often retained between the ends of a magnet pole and the outer periphery of the rotor core. These narrow sections are often called “bridges” or bridge areas. FIG. 1 shows a schematic cross sectional view of a conventional IPM motor 10 with magnets disposed in the rotor core. FIG. 2 is an enlarged view of a portion of the motor in FIG. 1. As depicted, the motor 10 includes a stator core 11 having a hollow cylindrical ring 20 and a core portion 15 formed inside the ring 20. The core portion 15 has slots 12 punched therethrough and coils 14 are wound around the slots 12. The motor 10 also includes a cylindrical rotor core 16 disposed on the inner side of the stator core 11, wherein a plurality of holes 18 are formed in the rotor core. Each hole 18 corresponds to a pole, extends through in the axial direction, and has a U-shaped cross section. Three permanent magnets 26 are inserted in each hole 18. Reference numeral 22 represents a magnetic-flux holding portion or center pole section that is located on the radially outward side of the hole 18. Numeral 24 represents a bridge that is disposed between the end of a magnet hole 18 and the outer periphery of the rotor 16. Reference numeral 28 denotes a rotor-shaft inserting hole.
During operation, the centrifugal force acting on the permanent magnets 26 and the centrifugal force acting on the center pole section 22 are concentrated in the bridges 24 of the rotor core 16. For this reason, the radial width of the bridges 24 must be large enough to maintain the required mechanical strength. The ring 23, which is formed of highly rigid nonmagnetic material, provides additional strength for the bridges 24. However, with this arrangement of magnets, the amount of magnetic flux leakage through the bridges 24 is a compromise with the mechanical strength of the rotor core under the centrifugal forces imparted by rotation. Two types of flux leakages occur through the bridge areas 24 and need to be reduced; 1) the flux leakage from the permanent magnets 26 needs to be reduced so that more of the magnet flux is allowed to link the stator core 11, thereby increasing the magnetic repulsion/attraction torque, 2) flux produced by the coils 14 and induced in the rotor core 16 at the direct-axis rotor position also needs to be minimal in leakage across the bridges 24 so as to increase the reluctance torque produced by the motor 10. A difficulty in the conventional motor 10 may be that the bridge portions 24 need to be made thick to meet the mechanical strength requirement at the expense of a higher flux leakage that leads to a lower torque production. Conversely, thinner bridges lead to a reduction in rotor strength limiting the speed capability of the motor 10. This trade-off relationship between the mechanical strength and magnetic flux leakage has limited the development of higher-speed, higher torque motors.
U.S. Pat. No. 6,906,444 discloses various types of rotors configured to address the trade-off issue. FIGS. 3 and 4 show schematic transverse cross sectional diagrams of rotor cores described in the '444 patent. The rotor core 34 in FIG. 3 includes a plurality of poles, wherein each pole has three trapezoidal shaped magnet holes 35 and three permanent magnets 36 inserted in the magnet insertion holes 35. Both the holes 35 and permanent magnets 36 have prismatic shapes. Between neighboring holes 35 there are disposed ribs 37 that prevent the centrifugal force acting on the permanent magnets 36 and center pole section 39 from being concentrated in the bridges 38, thereby enhancing the rotational speed limit without increasing the radial width of the bridges 38. However, the multiple magnet insertion holes 35 and ribs 37 increase magnetic flux leakage between neighboring poles and reduce torque production, i.e., a higher rotational speed may be obtained at the expense of torque reduction.
The rotor core 40 in FIG. 4 includes U-shaped permanent magnet insertion holes 43 and permanent magnets 44 inserted in the holes. The rotor core 40 also includes an annular nonmagnetic ring 42 that covers the outer peripheral portion of the rotor core. The ring 42, which is formed of highly rigid nonmagnetic material, is used in place of bridges or in addition to the bridges. Because the annular nonmagnetic ring 42 is fitted over the outer peripheral portion of the rotor core 40, the structure is able to resist the breakage of the rotor due to the centrifugal force acting on the magnets 44 and center pole section 46 during operation. Also, the magnetic flux leakage from the center pole section 46 is reduced, thereby making it possible to obtain a high magnetic flux density in the center pole section 46. As the size of the permanent magnets 44 can be made large and the magnetic flux density produced in the center pole section 46 as well as the saliency in the rotor core 40 can increase, the overall torque can be increased. However, the disadvantage of incorporating the nonmagnetic ring 42 is that it significantly increases the manufacturing cost. Also, the gap between the rotor core 40 and stator (not shown in FIG. 4) decreases and eddy current loss may increase if the ring 42 is metallic. Thus, there is a need for low cost motors with enhanced rotational speeds, power, and torque densities.